Vibration reduction is an important part of performance enhancement for machines having rotating components. For example, helicopter vibration reduction is an important part of aircraft maintenance and ride quality.
Traditionally, such vibration is reduced by controlling the mass of the rotors and the aerodynamic effect of the rotor blades. For example, the mass error of a helicopter rotor is typically detected using a single accelerometer mounted near the rotor mast. The aerodynamic error in forward flight is usually detected by measuring the vertical “hop” in an accelerometer mounted in the nose of the helicopter. However, since vibration is a convolution of multiple vibration sources such as mass error, track error and other contributing factors such as leading or lagging blades, it is useful to measure blade track and airframe vibration in multiple axis simultaneously to help determine what type of mass or aerodynamic adjustment can best reduce the overall vibration.
There are several traditional methods for checking the track of helicopter rotor blades in a hover. For example, one method uses a flag on a stick that is positioned near the rotors. As the rotors pass the flag, the blade tips strike the flag, leaving marks that show the relative position of the blades. This method can be time consuming and dangerous to the operator. Another method uses reflective blade tips or illuminated blade tips to make the tips of the blades viewable under certain lighting conditions. In yet another method, strobe lights are used to visualize the blades movement and track.
In another example, an optical blade tracker is used to detect the distance of individual blades using a parallax method. Such a method is described in U.S. Pat. No. 5,929,431, which is hereby incorporated in its entirety herein. In that system, two sensors are used to generate two optical detection fields. As the blade passes through the two detection fields, the time is measured between the interruptions of the two fields. If the rotor diameter, RPM, and chord of the blades are known, then the distance, and difference in distance can be calculated, the track error calculated, and corrections can be made to adjust the rotor.
However, in some current systems, the optical blade tracker is not always perpendicular to the rotor blades and adjustment calculations must be done to compensate for the angular error in the position of the optical tracker. For instance, the distance that the optical blade tracker is mounted below the rotor, the distance ahead of the main rotor, and the angle looking up toward the rotor, are all measurements that need to be determined prior to operating the optical blade tracker. This measurement of distances and angles can be problematic and time consuming. Errors in the measurements may cause errors in the track estimate and the rotor smoothing.
Additionally, current systems require attachment of multiple accelerometers around the airship, as well as one at the mast and one at the nose, and then running wires back to a central computer from each accelerometer for vibration analysis. These multiple accelerometers are located to maximize the translational vibration in the accelerometer. However, the attachment and complexity of the wiring systems can be problematic, as well as time consuming to attach and maintain.